The objective of the proposed investigations is to determine what is responsible for the dramatically reduced rate of inactivation of factor Xa by antithrombin III (even in the presence of heparin) when the factor Xa is associated with platelet membrane surfaces or factor Va and phospholipid bilayers. Identification of which interaction(s) between and among the components of the prothrombin activation complex and their relative quantitative importance for the protection is to be determined from the extent to which each component alone and the components together reduce the rate at which factor Xa is inactivated by antithrombin III. A second objective is to develop a quantitative (kinetic) description of the process of prothrombin activation as the first step to determining how it may be regulated. Of particular interest is determination of the importance of the rate of activation of factor V in regulation of prothrombin activation. Necessary to molecular interpretation of these investigations is determination of the binding constants for interaction between and among factor Xa, factor Va, phospholipid and calcium ions and the platelet membrane. Secondarily, the possible role of prothrombin as a competitor for the "active site" of factor Xa in the prevention of factor Xa inactivation by antithrombin III must also be ascertained. Because of the considerable confusion surrounding the significance of direct interaction of "heparin", affinity selected heparin molecular fragments and heparin like anionic polysaccharides with the antithrombin III versus coagulation proteases, a third aim of these studies is to determine if the direct interaction of various "heparins" with Factor Va, factor Xa and phospholipid significantly inhibits prothrombin activation or alters the extent to which factor Xa is protected from antithrombin III. Also to be investigated is the possibility that a relationship exists between formation of the proteolytically modified antithrombin, "heparin" interactions with the antithrombin and the activation system components and protection of factor Xa.